Diode

ABSTRACT

A diode is disclosed. One embodiment provides a semiconductor body having a front and a back, opposite the front in a vertical direction of the semiconductor body. The semiconductor body contains, successively in the vertical direction from the back to the front, a heavily n-doped zone, a weakly n-doped zone, a weakly p-doped zone and a heavily p-doped zone. In the vertical direction, the weakly p-doped zone has a thickness of at least 25% and at most 50% of the thickness of the semiconductor body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Utility Patent Application claims priority to German PatentApplication No. DE 10 2007 001 108.5-33 filed on Jan. 4, 2007, which isincorporated herein by reference.

BACKGROUND

Diodes, for example, power diodes, may include heavily p-doped layer, aweakly n-doped layer and a heavily n-doped layer. Such diodes are usedprimarily in powerelectronics, for example in high-voltage DCtransmission systems. They can have high reverse voltages of a fewkilovolts, e.g., more than 4000 V. It is known that such diodesgenerally experience a transient voltage overshoot in the anode/cathodevoltage when turned on with high current gradients.

The maximum transient voltage overshoot that occurs when the diode isturned on is usually set by a suitable choice of diode thickness or asuitable choice of doping for the weakly n-doped zone. In this case, areduction in the diode thickness or an increase in the dopingconcentration of the weakly n-doped zone results in lower overvoltages.

This measure, however, is accompanied by a reduction in the diode'sreverse voltage. Furthermore, an increase in the doping concentration ofthe weakly n-doped zone affects the diode's stability toward cosmicradiation.

SUMMARY

One embodiment relates to a diode that has a semiconductor body whichcontains, successively in a vertical direction, a heavily n-doped zone,a weakly n-doped zone, a weakly p-doped zone and a heavily p-doped zone.The thickness of the weakly p-doped zone is at least 25% and at most 50%of the thickness of the semiconductor body.

Within the context of the present disclosure, the term “thickness” meansdimension in a vertical direction.

The thickness of the weakly n-doped zone is particularly dependent onthe diode's reverse voltage which is to be achieved. By way of example,every 10 V of reverse voltage may require the weakly n-doped layer tohave a thickness of 1 μm. With these values, a diode with a reversevoltage of 4000 V means that the weakly n-doped layer has a thickness of400 μm. Accordingly, thicknesses of 450 μm or 500 μm are obtained forreverse voltages of 4500 V or 5000 V.

The net acceptor dose, i.e. the integral of the net dopantconcentration, in the weakly p-doped zone is in one example between1·10¹² cm⁻² and 2·10¹² cm⁻².

In one embodiment, the electrical field strength arising at the junctionbetween the weakly n-doped layer and the heavily n-doped layer when abreakdown voltage is applied is between 2·10⁴ V/cm and 1·10⁵ V/cm, forexample, 5·10⁴ V/cm.

To reduce the electrical field in the space charge zone, which is formedin the diode's blocking state between the weakly p-doped layer and theweakly n-doped layer, evenly in the edge region of the diode, thesemiconductor body may have an edge bevel which extends from the frontto beyond the pn junction formed between the weakly p-doped zone and theweakly n-doped zone.

The net dopant concentration of the weakly p-doped zone is, in oneembodiment, chosen to be approximately constant in the verticaldirection or falls from the surface to the depth with the smallestpossible gradient.

To further reduce the transient voltage overshoot which occurs when thediode is turned on and to achieve soft turn-off of the diode, the diodecan also be provided with a deep n-doped field stop zone which isarranged between the heavily n-doped zone and the weakly p-doped zone.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of embodiments and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments andtogether with the description serve to explain principles ofembodiments. Other embodiments and many of the intended advantages ofembodiments will be readily appreciated as they become better understoodby reference to the following detailed description. The elements of thedrawings are not necessarily to scale relative to each other. Likereference numerals designate corresponding similar parts.

FIG. 1 illustrates a cross section through diode in which a weaklyp-doped zone is arranged between the heavily p-doped zone and the weaklyn-doped zone.

FIG. 2 illustrates a diode as illustrated in FIG. 1 in which an n-dopedfield stop zone is additionally arranged between the heavily n-dopedzone and the weakly n-doped zone.

FIG. 3 illustrates the profile of the net dopant concentrations in thevertical direction of a first diode in comparison with a second diode.

FIG. 4 illustrates the time profile for the diode voltage of the firstdiode with the time profile for the diode voltage of the second diodebased on the prior art, in each case during the turn-on process andassuming a constant current rise.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments can be positioned in a number of differentorientations, the directional terminology is used for purposes ofillustration and is in no way limiting. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing detailed description, therefore, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

It is to be understood that the features of the various exemplaryembodiments described herein may be combined with each other, unlessspecifically noted otherwise.

FIG. 1 illustrates a cross section through a diode that has asemiconductor body 1 which includes successively in a vertical directionz: a heavily n-doped zone 5, a weakly n-doped zone 4, a weakly p-dopedzone 3 and a heavily p-doped zone 2.

The front 11 of the semiconductor body 1 has an anode metallization 6,and its back 12, opposite its front 11, has a cathode metallization 7.

In addition, in a lateral direction r which is at right angles to thevertical direction z, the edge region 13 of the semiconductor body 1 hasan optional edge bevel which extends from the front 11 to beyond the pnjunction 15 formed between the weakly p-doped zone 3 and the weaklyn-doped zone 4 as far as the back 12. The edge bevel is formed by virtueof the back 12 of a lateral edge 8 of the semiconductor body 1 enclosingan angle α of, in one embodiment, 30° to 50°.

In one embodiment, the edge bevel 8, the edge region 8 of thesemiconductor body 1 may also have a planar edge termination, forexample, one or more field rings with a respective field plate which isarranged on the front 11 and makes contact with the relevant field ring.

In the vertical direction z, the semiconductor body 1 has a thicknessd1. The thickness d3 of the weakly p-doped zone 3 in the verticaldirection z is at least 25% and at most 50% of the thickness dl of thesemiconductor body 1.

A reverse voltage across the diode is essentially reduced in the weaklyn-doped zone 4, which is why the latter's smallest thickness d4min mustnot be below a prescribed value if the diode is meant to withstand aparticular reverse voltage. In this case, the smallest thickness d4minshould be understood to mean the smallest local thickness of the weaklyn-doped layer 4 which in a vertical direction, i.e. at right angles tothe front and back 11 and 12, with only those regions of the weaklyn-doped layer 4 which are adjoined by semiconductor material in thevertical direction both above and below the weakly n-doped layer beingtaken into account. In the case of the diode illustrated in FIG. 1, thethickness of the weakly n-doped zone 4 is constant and hence identicalto the minimum thickness d4min if the edge region 13 is ignored.

Since every 10 volts of reverse voltage require the weakly n-doped zone4 to have a thickness of approximately 1 μm, a diode with a reversevoltage of 4 kV has a minimum thickness d4min of at least 400 μm.Accordingly, diodes with reverse voltages of 4.5 kV or of 5 kV haveminimum thickness d4min of at least 450 μm or at least 500 μm.

The net acceptor concentration of the weakly p-doped zone 3 is, in oneexample, between 1·10¹² cm⁻³ and 1·10¹⁴ cm⁻³, or in another example,between 5·10¹² cm⁻³ and 5·10¹³ cm⁻³.

The net acceptor dose of the weakly p-doped zone 3 is in one examplebetween 1·10¹² cm⁻² and 2·10¹² cm⁻².

The thickness d1 of the semiconductor body 1 in the vertical direction zis in one embodiment dimensioned such that a field strength of at least5·10⁴ V/cm is produced at the junction between the weakly n-doped zone 4and the heavily n-doped zone 5 at the diode's breakdown voltage. Thismeans that the diode is designed for the space charge zone to “punchthrough” the weakly n-doped zone 4.

To achieve an even reduction in the electrical field in a semiconductorbody 1 in the diode's blocking state in the edge region 13 and theregion close to the edge, the semiconductor body may have an edgetermination, for example an edge bevel 8. In this case, the edge bevel 8extends from the front 11 of the semiconductor body 1 to beyond the pnjunction 15 between the weakly p-doped zone 3 and the weakly n-dopedzone 4.

Instead of or in addition to an edge bevel 8, other edge terminations,for example field rings with or without field plates, may also beprovided on the front 11 of the semiconductor body 1.

As FIG. 2 illustrates, the diode may optionally have an n-doped fieldstop zone 9 which is arranged between the weakly p-doped zone 3 and theheavily n-doped zone 5. The field stop zone 9 can directly adjoin theheavily n-doped zone 5 in the lateral direction—as FIG. 2illustrates—or—what is not illustrated—can be at a distance therefrom inthe vertical direction z. In the latter case, there is then a section ofthe weakly n-doped zone 4 between the field stop zone 9 and the heavilyn-doped zone 5.

The field stop zone 9 illustrated in FIG. 2 is of simple contiguousdesign. In one embodiment, the field stop zone 9 may also be formed fromtwo or more subzones, however, which are at a distance from one anotherin the lateral direction r and/or the vertical direction z. In oneembodiment, the side of the field stop zone 9 which faces the front 11is at a greater distance from the front 11 in the edge region 13 of thesemiconductor body 1 than in the central region of the semiconductorbody 1.

In the vertical direction z, the weakly p-doped zone 3 has a net dopantconcentration N_(D) which is in one embodiment approximately constant orwhich has the smallest possible gradient in the vertical direction z,the net acceptor doping concentration in one example fallingmonotonously as the distance from the surface increases.

The net dopant concentration N_(D) in the region of the field stop zone9 is in one embodiment chosen to be greater than the net dopantconcentration N_(D) in the region of the weakly n-doped zone 4, but lessthan the net dopant concentration N_(D) in the region of the heavilyn-doped zone 5.

Besides having a reduced voltage overshoot upon turn-on in comparisonwith a diode based on the prior art, the additional weakly p-doped zone3 also has an affect on the edge termination of the diode if the latteris provided with a beveled edge 8 to improve edge blocking ability.

Another effect of the additional weakly p-doped zone 3 is when the diodeis turned off with high current gradients from the conductive state tothe blocking state. The holes which then flow away to the heavilyp-doped zone 2 from the charge carrier plasma at least partiallycompensate for the negative acceptor charges in the space charge zone ofthe heavily p-doped zone 2, and therefore ensure a reduction in theelectrical field strength and an accompanying reduction in the chargecarrier generation rate as a result of impact ionization processes. Forthe dynamic avalanche, there may even be a positive effect if the netacceptor dose of the weakly p-doped zone 3 in the vertical direction zis greater than approximately 1.3·10¹² cm⁻². This applies particularlyif the net dopant concentration N_(D) and the doping gradient in thevertical direction z of the semiconductor body 1 are set such that thechange to the blocking state in the weakly p-doped zone 3 dynamicallyproduces a relatively small gradient for the electrical field strengthin the vertical direction z.

By way of example, the weakly p-doped zone 3 can be fabricated by takinga semiconductor body 1 which has a weakly n-conductive base doping N_(D)and introducing aluminum from the front 11. To this end the front 11 mayinitially be coated with aluminum which can then be diffused into thesemiconductor body 1 to a depth td (see FIGS. 1 and 2) of between 25%and 50% of the total thickness d1, in one embodiment between 40% and 50%of the total thickness d1, in an insertion step.

In one embodiment, the method provides for aluminum to be introducedinto the semiconductor body 1 by ion implantation, in one embodiment,from the front 11, and then for an insertion step to be provided.

The n-doped field stop zone 9 may be fabricated by diffusing in sulfurand/or selenium from the back.

In one embodiment, the weakly p-doped zone 3 may also have a net dopantconcentration N_(D) which is constant or approximately constant in thevertical direction z. In this case, a semiconductor body 1 with a p-typebase doping may also be used to fabricate the diode. Deep n-type dopingprofile with a low net dopant concentration N_(D) and a small gradientfrom the net dopant concentration N_(D) in the vertical direction z isproduced in the semiconductor body by diffusing in sulfur and/orselenium and/or hydrogen from the back 12 of the semiconductor body 1.Hydrogen is diffused in from a plasma or a combination of protonirradiation followed by a heating step in which the semiconductor body 1is heated to temperatures of between 350° C. and 550° C., so thathydrogen-correlated donors are formed.

Proton irradiation allows soft turn-off of the diode.

The diode illustrated in FIGS. 2 and 3 is rotationally symmetrical aboutan axis of symmetry A-A′, i.e. it has a circular cross section in everysectional plane at right angles to the vertical direction z.

In one embodiment, the axis of symmetry A-A′ may also be a quaternaryaxis of symmetry, i.e. the cross section of the diode is square in everysectional plane at right angles to the vertical direction z.

FIG. 3 contrasts the profile of the net dopant concentration 20 of aconventional diode and the profile of the net dopant concentration 21 ofa diode having an additional weakly p-doped zone 8. Both diodes have thesame thickness d1 in the vertical direction z.

FIG. 4 contrasts the time profile 30 of the diode voltage U of a diodebased on a prior art and the time profile 31 of the diode voltage U ofan inventive diode having a weakly p-doped zone 8 as illustrated inFIG. 1. The diodes have the relevant doping profiles illustrated in FIG.3. For both diodes, the rise in the diode current over time I(t) hasbeen chosen to be constant and of the same magnitude.

It can be seen that with a conventional diode a negative voltage peakoccurs whose magnitude is greater than 600 V, whereas the magnitude ofthe corresponding negative voltage peak of a diode having a weaklyp-doped zone is only a little more than 300 V. Another effect is thatthe additional weakly p-doped zone 8 can bring about not only areduction in the transient voltage overshoot in the anode/cathodevoltage which occurs when the diode is turned on but also an increase inthe diode's breakdown voltage.

Thus, by way of example, a diode having a net dopant concentration 21 asillustrated in FIG. 3 has a breakdown voltage of 13.2 kV, whereas thebreakdown voltage of the conventional diode with a net dopantconcentration 20 as illustrated in FIG. 3 is just 11.5 kV.

The embodiments explained above are suitable particularly for diodeswith high reverse voltages of at least 4 kV, at least 4.5 kV or at least5 kV, for example.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. A diode comprising: a semiconductor body having a front and a back,opposite the front in a vertical direction of the semiconductor body,and which contains, successively in a vertical direction from the backto the front, a heavily n-doped zone, a weakly n-doped zone, a weaklyp-doped zone and a heavily p-doped zone, the weakly p-doped zone havinga thickness in the vertical direction which is at least 25 % and at most50 % of the thickness of the semiconductor body in the verticaldirection, wherein a net acceptor concentration in the weakly p-dopedzone is between 1·10¹² cm⁻³ and 1·10¹⁴ cm⁻³.
 2. The diode of claim 1,wherein the weakly p-doped zone has a thickness in the verticaldirection at least 40 % and at most 50 % of the thickness of thesemiconductor body in the vertical direction.
 3. The diode of claim 1,wherein the net acceptor dose in the weakly p-doped zone is between1·10¹² cm⁻² and 2·10¹² cm⁻².
 4. The diode of claim 1, wherein the netacceptor concentration in the weakly p-doped zone is 1 to 10 times thenet donor concentration of the n-doped zone.
 5. The diode of claim 1,wherein a breakdown voltage at which the electrical field strength atthe junction between the weakly n-doped layer and the heavily n-dopedlayer is at least 5·10⁴ V/cm.
 6. The diode of claim 1, wherein thesemiconductor body has an edge bevel on its heavily n-doped zone.
 7. Thediode of claim 1, wherein the net dopant concentration of the weaklyp-doped zone is approximately constant in the vertical direction.
 8. Thediode of claim 1, wherein with an n-doped field stop zone whose netdopant concentration is greater than the net dopant concentration of theweakly n-doped zone, whose net dopant concentration is less than the netdopant concentration of the heavily n-doped zone, and which is arrangedbetween the heavily n-doped zone and the weakly n-doped zone.
 9. Thediode of claim 1, which has a reverse voltage of at least 4 kV.
 10. Thediode of claim 1, which has a reverse voltage of at least 4.5 kV. 11.The diode of claim 1, which has a reverse voltage of at least 5 kV. 12.A semiconductor body comprising: a back; a heavily n-doped zone adjacentthe back; a weakly n-doped zone adjacent the heavily n-doped zone; aweakly p-doped zone adjacent the weakly n-doped zone; a heavily p-dopedzone adjacent the weakly p-doped zone; and a front adjacent the heavilyp-doped zone; wherein the weakly p-doped zone has a thickness in avertical direction that is at least 25 % and at most 50 % of thedistance between the front and back of the semiconductor body in thevertical direction; wherein a net acceptor concentration in the weaklyp-doped zone is between 1·10¹² cm⁻³ and 1·10¹⁴ cm⁻³.